Commercialisation of acidic geothermal wells by ph buffering

As. J. Energy Env. 2005, 6(03), 175-185 178The HCO3 - has an equilibrium pH of 8-10. This method had been overlooked from atheoretical standpoint. It is true that the injection of CO2 or the re-injection of NCG adjuststhe geofluid to a more stable non-corrosive pH of 8.35. However, when the geofluid isallowed to flow out of the well, the pressure drops, and the temperature remains high,resulting in a substantial decrease in the solubility of CO2. As the concentration of dissolvedCO2 drops, the natural buffering effect of CO2 is affected; as a result, the natural acidityof the geofluid makes this technique futile for adjusting the pH. This method had been triedin the Philippines but was eventually rejected.The technique ofcommercializing acidic wells through the use of acid resistant materialsof construction for the steam gathering system and well casing would entail high initialfixed cost that is not feasible in third world countries, like the Philippines. This wouldlikewise result in a high cost of power conversion of geothermal energy making it lessfavorable compared to other sources of energy.Materials and MethodologiesBuffer and Buffer CapacityA buffer, composed of a weak acid and its strong salt with a strong base or a weak base andits strong salt with a weak acid, has the ability to maintain the pH of a solution betweennarrow limits [11]. Therefore, a buffered solution will have no appreciable change in its pHupon dilution or addition of slight to moderate amounts of a strong acid or base. Buffercapacity is a measure of the ability of the buffer system to neutralize added strong acid orbase without a significant change in pH. The resistance to change in pH of a solution isgreatest when [acid] / [salt] =1.Therefore, buffer capacity is a maximum for buffer solutionscontaining equivalent amounts of a weak acid and its salt.As the ratio [acid]/[salt] changesfrom unity,either by decreasing or increasing the acid concentration or the salt concentration,the buffer capacity changes.A solution is generally considered to have a useful buffercapacity if the[acid] to [salt] ratio is within the range from 0.1 to 10;hence, effective withinthe range pK a ± 1 [11].New Buffering SolutionThe new buffer used in this investigation is ß - chloropropionic acid- sodium - ß-chloropropionate (BCPH- NaBCP) with a pKa = 4.11 [10] and a boiling point of 205 o C[7]. It has a high decomposition point in water (>200) via hydrolysis (Merck Index, 1989).This buffer system was chosen because the designed pH is within 4.11 ± 1 which is theeffective buffer range [11] and will not react with geofluid components.For more than three years now, an extensive study of applying pH buffering to solvecorrosion and silica-scaling problems encountered in geothermal systems [21] has beenconducted.Cabigon had successfully tested this method in the laboratory for silicadissolution without affecting low carbon steel casing.